Title

Authors

Date of this Version

5-2016

Comments

A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, Major: Earth and Atmospheric Sciences, Under the Supervision of Professors Qi Hu and Robert Oglesby. Lincoln, Nebraska: May, 2016

Copyright (c) 2016 Mengyuan Shang

Abstract

The purpose of this research is to describe and compare different roles of the Great Plains low-level jet (GPLLJ) in producing warm season precipitation over the central United States by model simulation. After going through 35 years’ (from 1979- 2013) NCEP North American Regional Reanalysis (NARR) data, year 2002 was selected for model simulation as it contained a “wet period” (May- June) and a “dry period” (July- August). The model simulation was done by using Weather Research and Forecasting (WRF) Regional Model. In this study, the GPLLJ was defined by the low-level wind at 925 hPa. The results showed the GPLLJ activities were stronger and more frequent in the “wet period” than in the “dry period”. The 2-6 days’ synoptic systems were also more active in the “wet period” than the “dry period”. During the 2002 warm season, the wind direction of the GPLLJ was usually northward. Two cases, one from each period, were selected to compare the different roles of the GPLLJ in producing precipitation over central U.S. In the case chosen from the “wet period”, there were two major effects of the GPLLJ on precipitation: 1) coupling with an upper-level jet streak (defined by the upper-level wind at 200 hPa) to create rising motion; 2) transportation of moisture into central U.S. The development of the GPLLJ and its effects on promoting precipitation were largely associated with synoptic systems. However, in the case from the “dry period”, the effect of the GPLLJ was more emphasized on the transportation of warm, moist air from the Gulf of Mexico. With absence of synoptic system activities, the lift mechanism was provided by the weak warm front at the nose of the GPLLJ.